Helium exhaust experiments on JET with Type I ELMs in H-mode and with Type III ELMs in ITB discharges

An analysis of helium exhaust experiments on JET in the MkII-GB divertor configuration is presented. Helium is pumped by applying an argon frost layer on the divertor cryo pump. Measurement of the helium retention time, tau(He)(*),, is performed in two ways: by the introduction of helium in gas puffs and measurement of the subsequent decay time constant of the helium content, tau(He)(d*); and by helium beam injection and measurement of the helium replacement time, tau(He)(r*). In ELMy H-mode, with plasma configuration optimized for pumping, tau(He)(d*) approximate to 7.2 x tau(E)(th) is achieved, where tau(E)(th) is the thermal energy replacement time. For quasi-steady internal transport barrier (ITB) discharges, the achieved tau(He)(r*) approximate to 4.1 x tau(E)(th) is significantly lower. The achieved helium recycling coefficient, confirmed by an independent measurement to be R-eff approximate to 0.91, is the same in both scenarios. None of the discharges are dominated by core confinement. The difference in tau(He)(*)/tau(E)(th) is instead due to the confinement properties of the edge plasma, which is characterized by Type I ELMs for the H-mode discharges studied, and Type III ELMs for the quasi-steady ITB discharges. This difference is quantified by an independent measurement of the ratio of the helium replacement time with a helium edge source to the energy confinement time.